Your MWI dialpeers will have an extension length associated like "888...." which is the 888 prefix for MWI followed by a four-digit extension. Since the 0 extension for the Operator does not have four digits the outdial from CUCE to turn MWI on will fail. You can test this will 'debug ccsip messages' (and maybe debug voice dialpeer) and leaving the Operator a voicemail.
Do you have a voice register dn with "number 0", and is associated with a phone? That is unusual. The more common practice is to have a regular extension on the phone number belonging to the operator and use voice translations to convert between 0 and that number. (Or, in the CUCE auto-attendant change the number mapped to the 0 keypress for Operator have a different target than 0).
If you want to stick with the 0 extension on a phone, you'd probably need another pair of dialpeers with the specific MWI On and MWI Off codes specific to the operator. In the 888 example, the dialpeer would be 8880$ or the like to be as exact as possible.
I hope this is helpful.
Maren
